The invention relates to a screw for a solid-bowl screw-type centrifuge and to a method of extracting oil by means of a solid-bowl screw-type centrifuge.
A method which has been particularly effective for the extraction of olive oil is known from European Patent Document EP 0 557 758. In this process, a two-phase separation is carried out in which the oil is separated directly from the solids/water mixture.
The efficiency of this known method is very good per se.
Nevertheless, it is desirable to lower the residual oil content in a rape in order to increase the economic efficiency of the oil extraction.
The present invention provides for, on the one hand, a screw for a solid-bowl screw-type centrifuge and a process for extracting oil from fruit or seed.
The present invention provides for a screw or kneading screw for a solid-bowl screw-type centrifuge which has at least one screw blade and at least one screw blade segment in a delivery path in sections or areas between adjacent screw spirals. In addition, the at least one screw blade is preferably provided with recesses which are constructed such that centrifuged material can flow through between adjacent screw spirals.
With respect to the method of separating or extracting oil, it was found to be particularly favorable for the oil, as a liquid phase, to be extracted directly in a two-phase separating process. The oil is extracted as a liquid or first phase from a second or mixed phase which may include a mixture of water and solids. Thus, seeds or reduced fruit, such as olives or avocados, are first guided into a solid-bowl screw-type centrifuge through a first portion of a separating zone having at least one screw blade with one or more screw spirals in a cylindrical section of the centrifuge. The at least one screw blade is preferably constructed without any recesses in a delivery path area between the screw spirals and, preferably, no blade segments are constructed in the delivery path. Subsequently, a passing takes place into a second portion of the separating zone in which recesses are constructed in the at least one screw blade, and blade segments are constructed the delivery path. Then the solids and the water are conveyed past a retarding plate or disk, which acts as a barrier to the oil, from the separating zone into a conically tapering section or dry zone of the screw and then out of the centrifuge. The oil is conveyed in an opposite direction out of the centrifuge.
Also, by use of the screw according to the present invention, a three-phase oil extraction process, which is still occasionally used, can be improved. In this case, oil is separated or extracted as a liquid or first phase, in a three-phase separating process, from a second phase comprising water and a third phase comprising solids. The process occurs as follows:                the reduced fruit, such as olives or avocados or seeds are first guided into a solid-bowl screw-type centrifuge through a first portion of a separating zone having at least one screw blade with one or more screw spirals in a cylindrical section of the centrifuge. The at least one screw blade is preferably constructed without recesses, and preferably with no blade segments constructed in the delivery path between the screw spirals,        then, a passing takes place into a second portion of the separating zone, in which recesses are constructed in the at least one screw blade and blade segments are constructed in the delivery path,        then the three phases, water, solids and oil are guided/delivered out of the centrifuge essentially separately. The water and oil may exit at different levels toward a cylindrical end of the centrifuge and the solids may exit toward a conical end of the centrifuge.        
By use of the screw according to the present invention, the economic efficiency of the oil extraction can be increased considerably. In this regard, reference is particularly made to tests explained herein and whose results are shown in FIGS. 4 and 5. The screw of the present invention can also be retrofitted without any problem into existing centrifuges. The screw according to the present invention is particularly suitable for an application in a process for extracting oil from fruit and seeds and for a better draining of water and/or separating of oil from mashes of organic materials (such as seed mash, fruit flesh mash, animal tissue, such as fish, egg, fatty tissue cells).
According to the present invention, a combination of recesses and blade segments are provided. The blade segments and the recesses preferably are constructed such in the axial direction that the recesses each form ducts extending in the axial direction (and/or at an angle or in a zigzag-type manner with respect to the center axis y), in which ducts the blade segments stand.
Also according to the present invention, the blade segments and the recesses may be constructed only in the cylindrical section of the screw body and a retarding disk may be provided in the conical section of the screw, particularly in the two-phase separation.
According to the prior art, solid-bowl screw-type centrifuges are known, in which recesses are provided in the screw blade. For example, see German Patent Document DE 41 32 693 A1. However, according to the present invention, the simple providing of such recesses is not sufficient to obtain a significant increase in efficiency. On the contrary, an increase in efficiency can be achieved when, in addition to recesses, provided in a center of the delivery path between adjacent screw spirals, the blade segments are also constructed.
It is also known to construct blade-segment-type screw spirals, as is shown, for example, in International Patent Document WO 97/23295. Those blade segments extend into the conical section, which is not favorable. In addition, those blade segments are distributed on the circumference of the screw body over its entire area, which was also found to be not favorable. In addition, it is not that additional blade segments are set up in the delivery path between the screw spirals, but the blade segments themselves form the screw spirals. Also, by use of this prior art screw, no satisfactory economic efficiency can be achieved when extracting olive oil.
According to the present invention, the blade segments in the delivery path may be constructed such that they extend into an area where solids are present, such as a solids area. However, there is an exterior area of, for example, approximately 25 mm that is preferably not reached by the blade segments, because relatively completely de-oiled solids and permanently discharged solids are already present in this exterior area.
Measuring results indicate that the screw according to the present invention leaves approximately 1 to 1.5% less oil in a discharged solids sludge. During an olive oil extraction campaign, this corresponds to a financial savings of approximately DM 300,000.00 to 500,000.00 per centrifuge machine.
The screw of the present invention may operate in an area of moist orujo or rape, because in that area a special separation of oil can be achieved by means of the blade segments.
By use of the present invention, a solids mash can be fed into a bowl or drum preferably by way of a rectangular tube. The rectangular tube must be so long that the entering mass or mash to be centrifuged is charged or forced through an oil layer while being protected in order not to mix with the oil layer at a later time.
In a filled centrifuge machine, an oil separation area may occur rather close to the screw body, for instance, at a distance of approximately 10, 20 . . . , to 40 to 50 mm. Fresh oil, as a distinct phase, can generally be recognized approximately in the range of 20 to 30 mm outside or away from the screw body. A distinct separating line usually exists here. The range of the oil separation area may vary with different centrifuges.
Charged solids, as part of a fed suspension, will therefore fill the centrifuge to such an extent that the latter is filled to the oil separation area (approximately 10-50 mm outside the screw body) with solids suspensions. The reason is that, as a rule, only so little water is in the orujo or rape mass that no water or only an extremely small layer of free water is formed between the oil and the solids suspension. In this case, the solids are dryer on the outside than on the inside or, in other words, a fraction of dry substance on the drum side is much higher than a fraction of dry substance toward the interior.
In the area of the recesses and blade segments, the solids suspension, just like the oil and an emulsion situated in-between, experiences three axial speeds particularly in a kneading area of the blade segments, from the screw body to an outside radial end of the blade segment.
Thus, a normal axial speed exists in the area of residual wall pieces or sections of the screw spirals. In contrast, in the area of the recesses, the axial speed is essentially zero. However, the axial speed in the area of the actual blade segments in the delivery path may amount to five times the normal speed. As a result, an elastoviscous sludge is deformed, compressed and relaxed in a standing solids area adjacent a surface of the drum.
In the area of the leading blade segments, for example x+1, x+2, x+3, x+4, the solids are additionally axially compressed. In the area of the recesses, they are then relaxed. This has the effect of pressure increases and relaxations. A setting-free or separation of the oil essentially takes place in a relaxation area and the extraction of oil is therefore more effective than without such relaxation areas.
In a rearward area, the screw body preferably has a cylindrical section and, in its adjoining forward section, a section which tapers essentially conically in a uniform or non-uniform—for example, stepped manner. The recesses and blade segments are constructed only in the area of the cylindrical section.
In the cylindrical section, the screw body preferably first has at least one screw spiral which is constructed without recesses as well as without blade segments and which is followed by additional screw spirals which are provided with the recesses and blade segments.
It is also conceivable that optional oil drainage ducts are constructed preferably in the first screw spiral.
The recesses preferably have a residual section of the screw blade on the circumference of the screw body.
Relative to one or several screw spirals, the blade segments may be uniformly, or may be non-uniformly, distributed on the circumference of the screw body.
The area of the recesses may amount to approximately 25-60%, preferably approximately 40-50% of the screw spiral area.
The recesses in the screw blades may be constructed such that they radially project at least beyond the solids area (for example, 70-95%, preferably 70-100% of the screw blade height).
The height of the blade segments may be approximately 0-30% lower than the height of the screw blade.
The blade segments may be constructed as rectangular metal plates. Trapezoidal, rounded elements and/or elements shaped to be tapering or widening and extending from the screw body radially outward or to the outside are also conceivable.
Other aspects and novel features of the present invention will become apparent from the following detail description of the invention when considered in conjunction with the accompanying drawings.